Two-dimensional photonic crystal surface emitting laser

ABSTRACT

To provide a two-dimensional photonic crystal surface emitting laser capable of improving characteristics of light to be emitted, in particular, optical output power. The two-dimensional photonic crystal surface emitting laser  10 X includes: a two-dimensional photonic crystal  123  including a plate-shaped base member  121  and modified refractive index regions  122  where the modified refractive index regions  122  have a refractive index different from that of the plate-shaped base member  121  and are two-dimensionally and periodically arranged in the base member; an active layer  11  provided on one side of the two-dimensional photonic crystal  123 ; and a first electrode  15 A and a second electrode  16  provided sandwiching the two-dimensional photonic crystal  123  and the active layer  11  for supplying current to the active layer  11 , where the second electrode  16  covers a region equal to or wider than the first electrode  15 A, wherein the first electrode  15 A is formed so as to supply the current to the active layer  11  with a different density depending on the in-plane position on the first electrode  15 A.

TECHNICAL FIELD

The present invention relates to a semiconductor laser, and in particular, to a two-dimensional photonic crystal surface emitting laser for amplifying light using a two-dimensional photonic crystal.

BACKGROUND ART

Semiconductor lasers have many advantages, such as small, inexpensive, low power consumption and long service life, and they have been widespread in a wide variety of fields, such as light sources for optical recording, light sources for communication, laser displays, laser printers, and laser pointers. Meanwhile, in the field of laser processing or laser machining, lasers having an optical output power exceeding at least 1 W are required. However, semiconductor lasers currently in actual use do not achieve this output due to the reasons later described. Thus, currently in the laser machining field, gas lasers such as carbon dioxide gas lasers are used instead of semiconductor lasers.

The reason why the optical output powers of semiconductor lasers currently in use are low is as follows. To increase the optical output power of the semiconductor laser, it is preferable that the laser beam emitted from the laser element has a large sectional area (emission area). On the other hand, to increase the machining accuracy, it is preferable that the laser beam to be spotted onto a workpiece has a small sectional area (spot area).

Consequently, ideally speaking, it is desirable that the laser beam emitted from a laser source reach the workpiece without broadening. However, in the semiconductor laser, as the emission area increases, the spread angle of the laser beam increases and the wavefront distortion of laser light occurs. If the wavefront distortion of the laser light occurs, it is difficult to obtain a small spot area even when light is focused using an optical system. It is thus difficult for the semiconductor lasers currently in use to achieve the optical output power of 1 W or higher while achieving a small spread angle.

Recently, Noda and Liang among the present inventors have developed a two-dimensional photonic crystal surface emitting laser having the following characteristics: an optical output power of 1.5 W and a beam spread angle of 3° or less (Non-Patent Literatures 1 and 2). The two-dimensional photonic crystal surface emitting laser includes: a two-dimensional photonic crystal including a plate-shaped base member and modified refractive index regions where the modified refractive index regions have a refractive index different from that of the plate-shaped base member and periodically arranged in this member; and an active layer. In the two-dimensional photonic crystal surface emitting laser, when electric current is injected into the active layer, only light having a predetermined wavelength corresponding to the periodicity of modified refractive index regions is amplified among light generated in the active layer, and is caused laser oscillation, whereby a laser beam is emitted in the direction perpendicular to the two-dimensional photonic crystal. The two-dimensional photonic crystal surface emitting laser emits light (surface emission light) from a certain area in the two-dimensional photonic crystal. Consequently, this laser has a larger emission area than that of an end surface emission semiconductor laser, which can enhance the output power and reduce the spread angle. Conventionally, various two-dimensional photonic crystals including modified refractive index regions with different planer shapes (circle, regular triangle, etc.), arrangements (triangular lattice, square lattice, etc.) or other parameters have been known. In the two-dimensional photonic crystal surface emitting lasers described in Non-Patent Literatures 1 and 2, the modified refractive index regions having a planar shape of right triangle are arranged on the lattice points of a square lattice parallel to the orthogonal sides of the modified refractive index region, whereby the optical output power is increased from the conventional two-dimensional photonic crystal surface emitting lasers.

CITATION LIST Non Patent Literature

-   [Non Patent Literature 1] Kazuyoshi Hirose and other five authors,     “Watt-class high-power, high-beam-quality photonic-crystal lasers”,     Nature Photonics (UK), 8, pp. 406-411, issued on Apr. 13, 2014. -   [Non Patent Literature 2] National university corporation Kyoto     University, and Hamamatsu Photonics K.K. “Watt-class high-power     photonic-crystal lasers: achieved leading the world—watt-class high     output with high-beam-quality achieved through surface emitting     laser first in the world” [online] National university corporation     Kyoto University's web Page [retrieved on Aug. 11, 2014], from the     Internet Apr. 10, 2014.

SUMMARY OF INVENTION Technical Problem

To improve the characteristics of the two-dimensional photonic crystal surface emitting laser, conventional researches have focused only on the configuration of two-dimensional photonic crystals. However, the laser of a semiconductor laser is generated by a cooperating action between light emission from the active layer and amplification in the two-dimensional photonic crystal. Conventionally, no research has focused on this relationship.

An object to be achieved by the present invention is, taking account of the cooperating action between light emission from the active layer and amplification in the two-dimensional photonic crystal, to provide a two-dimensional photonic crystal surface emitting laser that improves the characteristics of light to be emitted, in particular, the output power.

Solution to Problem

The present inventors scrutinized the configuration of an electrode for injecting charges into the active layer in the two-dimensional photonic crystal surface emitting laser. The inventors have found, and achieved the present invention based on, the fact that providing distribution in the charge (carrier) density of the active layer depending on the in-plane position brings about an amplification effect different from that in a case where the charge density of the active layer is uniform in the two-dimensional photonic crystal.

A two-dimensional photonic crystal surface emitting laser according to the present invention aimed at solving the aforementioned problem is a two-dimensional photonic crystal surface emitting laser including:

a two-dimensional photonic crystal including a plate-shaped base member and modified refractive index regions where the modified refractive index regions have a refractive index different from that of the plate-shaped base member and are two-dimensionally and periodically arranged in the base member;

an active layer provided on one side of the two-dimensional photonic crystal; and

a first electrode and a second electrode provided sandwiching the two-dimensional photonic crystal and the active layer for supplying current to the active layer, where the second electrode covers a range equal to or wider than the first electrode,

wherein the first electrode is formed so as to supply the current to the active layer with a different density depending on an in-plane position on the first electrode.

Typically, in the two-dimensional photonic crystal surface emitting laser, in order to let light out in a plane direction, two electrodes provided sandwiching the two-dimensional photonic crystal and the active layer are formed such that one on a light emission side is made of a transparent electrode and the other is made of a non-transparent (reflective) electrode, or the electrode on the light emission side is formed smaller than the other electrode. The active layer and the two-dimensional photonic crystal are provided in a range equal to or wider than these electrodes (wider than the larger electrode when these electrodes differ in size). Consequently, current (charges) flowing between these electrodes is injected into a partial region of the active layer (hereinafter, called “charge injection region”). In a case where one of the electrodes is smaller than the other electrode, the charge injection region is generally similar to the smaller electrode, depending on the areas of the electrodes and the distance between the electrodes.

In the two-dimensional photonic crystal surface emitting laser according to the present invention, the smaller first electrode (narrower range) supplies the active layer with current having different densities depending on the in-plane position, thereby allowing the charge densities in the charge injection region in the active layer to have different values depending on the in-plane position. Consequently, a distribution of light emission intensities in the active layer is formed depending on the in-plane position. In the two-dimensional photonic crystal, this light emission intensity distribution causes an amplification effect different from that in a case where the charge density in the active layer is uniform.

As one of measures for increasing the optical output power of the two-dimensional photonic crystal surface emitting laser, a configuration where the first electrode is formed so as to supply the current to the active layer where the distribution has a highest density at the center of the in-plane position may be adopted. Consequently, in the charge injection region in the active layer, light emission having the maximum intensity distribution at the center occurs. Amplification of such emitted light in the two-dimensional photonic crystal facilitates laser oscillation in a fundamental mode, and can reduce useless laser oscillation in higher modes. Consequently, the optical output power as a whole can be increased.

Contrary to the aforementioned example, in a case of selectively oscillating laser in the higher modes, current having high density distribution at positions other than the center in the in-plane position on the first electrode may be supplied to the active layer.

To supply the active layer with current having different densities depending on the in-plane position on the first electrode, the first electrode may adopt a configuration where the conductor is formed to have a mesh shape. In the first electrode made of such a mesh-shaped conductor, the thicknesses or intervals of the lines of the mesh, for example, may be set to be different depending on the position to make the conductor area ratio in the meshes to have different values depending on the position. This allows formation of current density distribution depending on the in-plane position on the first electrode.

To form the global current distribution over the entire first electrode by the mesh-shaped first electrode, it is desirable not to form local current distribution depending on the thicknesses and intervals of the lines of mesh. Thus, the intervals between the mesh lines are required to be 1.4 times or less as wide as the current spreading L_(c) in the active layer. On the other hand, to reduce the current density in a case where the locally periodic distribution of current is permitted, or to form such periodic distribution, the intervals of the mesh lines may be 1.4 times wider or more. Here, the current spreading L_(c) in the active layer is a value representing the size of a region where current flowing from one point in the first electrode toward the second electrode spreads in the plane of the active layer, and is defined by the distance from the center to the end of the region. The current spreading L_(c) is substantially equal to the carrier distribution length in the active layer.

The difference in current density depending on the in-plane position in the mesh-shaped first electrode can be formed by a mesh having regions with different area ratios of the conductor in the mesh, for example. In this case, the mesh is formed so as to reduce gradually the area ratio of each of the regions as the position is apart from the center of the in-plane position of the first electrode, thereby causing emission of light having intensity distribution being the maximum at the center in the charge injection region in the active layer. This emission of light is amplified in the two-dimensional photonic crystal, thereby facilitating occurrence of laser oscillation in the fundamental mode. Furthermore, in this case, the region including the center may constitute only of a conductor.

Alternatively, the difference in current density depending on the in-plane position in the mesh-shaped first electrode can be formed by varying the thicknesses and intervals of the mesh lines so as to change continuously the area ratio of the conductor in the mesh depending on the position. In this case, the mesh is formed so as to reduce gradually the area ratio of the conductor in the mesh as the position is apart from the center of the in-plane position of the first electrode, thereby causing emission of light having intensity distribution being the maximum at the center in the charge injection region in the active layer. This emission of light is amplified in the two-dimensional photonic crystal, thereby facilitating occurrence of laser oscillation in the fundamental mode.

In a case of selectively oscillating laser in a higher mode in such a mesh-shaped first electrode, the area ratio of the conductor at a position other than the center of the in-plane position of the first electrode may be set to the maximum.

As another specific form of the first electrode for supplying the active layer with current having densities different depending on the in-plane position, one that includes ring-shaped conductors arranged concentric-circularly, and a connective part for electrically connecting the ring-shaped conductors to each other can be taken. The center of the concentric circles may be a space without the conductor. Alternatively, a non-ring-shaped (typically, circular) conductor may be arranged at the center. In this specific form, by varying the width and/or the interval of the ring-shaped conductors depending on the distance from the center, concentric-circular current density distribution can be formed. To facilitate occurrence of laser oscillation in the fundamental mode, the widths of the ring-shaped conductors may be narrowed and/or the intervals may be broadened as the position is apart from the center of the in-plane position on the first electrode. In a case of selective oscillation of the higher mode, another width and/or interval can be adopted. In a manner similar to that of the intervals between the mesh lines in the mesh-shaped first electrode, it is desirable that the intervals of the ring-shaped conductors be narrow to form a global current distribution over the entire first electrode. To reduce the current density or form local and periodic distribution of current, it is desirable that the interval be wide.

As another specific form of the first electrode for supplying the active layer with current having different densities depending on the in-plane position, a form where the first electrode is divided into regions including sub-electrodes electrically insulated from each other can be taken. In this specific form, power sources different from each other are connected to the respective sub-electrodes, and voltages supplied between the sub-electrodes and the second electrode are adjusted, thereby allowing the current densities in the respective regions to be controlled.

Advantageous Effects of Invention

According to the present invention, the first electrode is formed so as to supply the active layer with current having different densities depending on the in-plane position, thereby allowing distribution of densities of charges in the charge injection region in the active layer to be formed different depending on the position. This formation can improve the characteristics, particularly the optical output power, of laser light to be amplified in the two-dimensional photonic crystal and be emitted to the outside.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A and FIG. 1B are perspective views showing an embodiment of a two-dimensional photonic crystal surface emitting laser according to the present invention.

FIG. 2A and FIG. 2B are a perspective view and a plan view respectively showing an example of a two-dimensional photonic crystal in the two-dimensional photonic crystal surface emitting laser of the present embodiment.

FIG. 3 is a schematic diagram showing a charge injection region in the active layer of the two-dimensional photonic crystal surface emitting laser of the present embodiment in contrast with two-dimensional photonic crystal.

FIG. 4 is a plan view showing a configuration of a first electrode in a two-dimensional photonic crystal surface emitting laser of a first embodiment.

FIG. 5A and FIG. 5B are graphs showing charge density distributions of the active layer according to the first embodiment, FIG. 5A shows a central part of the active layer and FIG. 5B shows a peripheral part of the active layer.

FIG. 6 is a plan view showing the central part and the peripheral part of the active layer in the first embodiment.

FIG. 7A and FIG. 7B are graphs showing results of calculating threshold gain differences Δα in the first embodiment, FIG. 7A shows the difference depending on the ratio L_(i)/L of the length L_(i) of one side of a first conductive region to the length L=200 μm of one side of the first electrode, and FIG. 7B shows the difference depending on L in a case of L_(i)/L=0.5.

FIG. 8A to FIG. 8E are graphs each showing a result of calculating the difference of the charge density distribution of the active layer depending on the interval L₂ between mesh lines of the first electrode in the first embodiment.

FIG. 9A to FIG. 9F are graphs each showing a result of calculating the difference of the charge density distribution of the active layer depending on the width L₁ of the mesh line of the first electrode in the first embodiment.

FIG. 10A to FIG. 10C are plan views showing three modified examples of first electrodes in the first embodiment.

FIG. 11 is a plan view showing another modified example of the first electrode in the first embodiment.

FIG. 12A and FIG. 12B are graphs showing results of calculating threshold gain difference Δα in the example of FIG. 11, FIG. 12A shows the difference depending on the ratio w_(p)/L of the width w_(p) of the Gaussian distribution of charge density to the length L=200 μm of one side of the first electrode, and FIG. 12B shows the difference depending on L in a case of w_(p)/L=0.25.

FIG. 13 is a plan view showing another modified example of the first electrode in the first embodiment.

FIG. 14 is a plan view showing a configuration of a first electrode in a two-dimensional photonic crystal surface emitting laser of a second embodiment.

FIG. 15 is a plan view showing a configuration of a first electrode in a two-dimensional photonic crystal surface emitting laser of a third embodiment.

FIG. 16 is a plan view showing a modified example of the first electrode in the two-dimensional photonic crystal surface emitting laser of the third embodiment.

DESCRIPTION OF EMBODIMENTS

Embodiments of two-dimensional photonic crystal surface emitting lasers according to the present invention are described with reference to FIG. 1 to FIG. 16. Hereinafter, roughly three embodiments are described. First, a configuration common to the embodiments is described. Subsequently, the configurations characteristic to the respective embodiments are described mainly on the configuration of a first electrode.

EMBODIMENTS Configuration Common to Embodiments

A two-dimensional photonic crystal surface emitting laser 10X of each embodiment has a configuration in which a first electrode 15X, a first cladding layer 141, an active layer 11, a spacer layer 13, a two-dimensional photonic crystal layer 12, a second cladding layer 142, and a second electrode 16 are stacked in this order (FIG. 1). The order of the active layer 11 and the two-dimensional photonic crystal layer 12 may be inverted from the above order. For the sake of convenience, FIG. 1 shows the first electrode 15X on the upper side while showing the second electrode 16 on the lower side. However, the orientation of the two-dimensional photonic crystal surface emitting laser 10X of each embodiment in use is not limited to that shown in this diagram. Hereinafter, the configurations of the layers and electrodes are described.

The active layer 11 receives charges injected from the first electrode 15X and the second electrode 16 to emit light having a predetermined wavelength band. As the material of the active layer 11, an InGaAs/AlGaAs multiple-quantum well (light emission wavelength band: 935 to 945 nm) is used in the present embodiment. However, the material is not limited to this in the present invention. The active layer 11 has a square shape with a thickness of approximately 2 μm, and a side of the square has the same length as or a longer than that of the second electrode 16 or 16A described later. The dimension of the active layer 11 is not limited to this in the present invention, and another shape of layer, such as a circular, or hexagonal may be employed.

The two-dimensional photonic crystal layer 12 has, for example as shown in FIG. 2, arrangement in which modified refractive index regions 122 having a refractive index different from that of a plate-shaped base member 121 are periodically arranged on the base member 121. The material of the base member 121 is GaAs in the present embodiment, but is not limited to this in the present invention. The modified refractive index regions 122 are air holes (air or vacuum) in the present embodiment. Alternative made of a material (having refractive index) different from that of the base member 121 may be used. In the present embodiment, the modified refractive index regions 122 having a right-triangular plan shape are arranged on the respective lattice points of a square lattice parallel to the orthogonal sides. The lattice constant a of the square lattice was set to 287 nm corresponding to the wavelength in the light emission wavelength band in the active layer 11 in consideration of the refractive index in the two-dimensional photonic crystal layer 12. Alternatively, another configuration may be employed where the planer shapes of the modified refractive index regions 122 are circular, regular-triangular or the like, and the arrangement of the modified refractive index regions 122 is a triangular lattice. The planar shape of the base member 121 is the same as that of the active layer 11, and the thickness is approximately 300 nm. In FIG. 2, vertical six and horizontal six modified refractive index regions 122 are drawn. In actuality, the modified refractive index regions 122 more than those in the diagram are provided.

The spacer layer 13 that is not an essential component in the present invention is provided to connect the active layer 11 and the two-dimensional photonic crystal layer 12 that are made of different materials. The material of the spacer layer 13 is AlGaAs in the present embodiment. However, the material is appropriately changed in accordance with the materials of the active layer 11 and the two-dimensional photonic crystal layer 12.

The first electrode 15X has a configuration specific for each embodiment. The detail of the configuration of the first electrode 15X is described in each embodiment. In FIG. 1, the detailed configuration of the first electrode 15X is eliminated, and only the external shape is drawn. Here, the material and the overall size of the first electrode 15X common to the embodiments are described. The material of the first electrode 15X is a p-type semiconductor in the present embodiment, and is non-transparent for laser light (a wavelength of 940 nm in vacuum in the present embodiment) emitted from the two-dimensional photonic crystal surface emitting laser 10X. The entire of first electrode 15X has a square shape with the length L of one side being approximately 200 μm, and is smaller than the active layer 11 and the two-dimensional photonic crystal 123. Around the first electrode 15X, a reflective layer (not shown) made of metal non-transparent for laser light is provided via an insulator. The reflective layer functions to cooperate with the first electrode 15X so as to reflect laser light generated by the two-dimensional photonic crystal surface emitting laser 10X and to emit the light to the outside from the second electrode 16.

In the example shown in FIG. 1A, the second electrode 16 is made of indium tin oxide (ITO) that is an n-type semiconductor transparent for the laser light. However, the material is not limited to this in the present invention, and indium zinc oxide (IZO) may be used instead, for example. The second electrode 16 has a square shape with each side having a length of approximately 800 μm, and has plan dimension equal to or slightly smaller than those of the active layer 11 and the base member 121 of the two-dimensional photonic crystal layer 12. Instead of adoption of such a transparent electrode, a second electrode 16A shown in FIG. 1B may be used. The second electrode 16A has a configuration with a plate-shaped square member made of metal non-transparent for laser light, and a center part of the member is cut out to form a square shape. The hollow part after cutting the plate-shaped member is called a window 161A, and the remaining plate-shaped member is called a frame 162A. The square of the plate-shaped member (outside of the frame 162A) has a side of 800 μm. The square of the window 161A has a side of 600 μm. In this example, laser light generated by the two-dimensional photonic crystal surface emitting laser 10X passes through the window 161A and is emitted to the outside.

The first cladding layer 141 and the second cladding layer 142 that are not essential components in the present invention function to connect the first electrode 15X and the active layer 11, and the second electrode 16 and the two-dimensional photonic crystal layer 12, respectively, and to facilitate current injection from the first electrode 15X and the second electrode 16 into the active layer 11. To achieve these functions, a p-type semiconductor was used as the material of the first cladding layer 141, and an n-type semiconductor was used as the material of the second cladding layer 142. The first cladding layer 141 has a two-layer structure consisting of a layer made of p-GaAs and a layer made of p-AlGaAs in order from the first electrode 15X side; likewise, the second cladding layer 142 has a two-layer structure consisting of a layer made of n-GaAs and a layer made of n-AlGaAs in order from the second electrode 16 side (neither of the two-layer structures are not shown). Also as to these first cladding layer 141 and second cladding layer 142, the materials are not limited to the materials described above in the present invention. The plan dimensions of the first cladding layer 141 and the second cladding layer 142 are the same as those of the active layer 11 and the base member 121 of the two-dimensional photonic crystal layer 12. The thickness of the first cladding layer 141 is 2 μm, and the thickness of the second cladding layer 142 is 200 μm. Thus, the active layer 11 is much nearer to the first electrode 15X than the second electrode 16. Accordingly, a charge injection region 111 (FIG. 3) in the active layer 11 has a planar shape and size similar to those of the first electrode 15X. The two-dimensional photonic crystal 123 is larger than the first electrode 15X. Thus, this crystal is larger than the charge injection region 111 of the active layer 11 (FIG. 3).

The operation of the two-dimensional photonic crystal surface emitting laser 10X is described. A predetermined voltage is applied between the first electrode 15X and the second electrode 16. A method of applying the voltage is different depending on the mode of the first electrode 15X of each embodiment, and the detail thereof is described in each embodiment. Through application of the voltage, current from both electrodes are injected into the charge injection region 111 of the active layer 11. Thus, charges are injected into the charge injection region 111 to cause emission of light having a wavelength in a predetermined wavelength band from the charge injection region 111. The charge density distribution and light emission intensity distribution in the charge injection region 111 are described in each embodiment. Among the emission of light thus caused, light having a wavelength corresponding to the lattice constant a of the square lattice is selectively amplified in the two-dimensional photonic crystal 123, and causes laser oscillation. The oscillating laser light is emitted from the second electrode 16 side to the outside.

Hereinafter, each embodiment is described mainly on the characteristic configuration.

First Embodiment—Mesh-Shaped Electrode

In a two-dimensional photonic crystal surface emitting laser of a first embodiment, a first electrode 15A having a configuration shown in FIG. 4 was used. The first electrode 15A has a square shape in an overall view, and includes two regions that are a square-shaped first conductive region 15A11 formed in the vicinity of the center, and a second conductive region 15A12 formed around the first conductive region 15A11. The first conductive region 15A11 is made of a uniform conductor (p-type semiconductor). Meanwhile, the second conductive region 15A12 has a mesh-shaped conductor. Areas between the mesh lines 15A2 are filled with inter-line regions 15A3 made of an insulator. SiN was used as the material of the inter-line regions 15A3. The conductor of the first conductive region 15A11 and the conductor constituting the mesh lines 15A2 in the second conductive region 15A12 are integrally formed and electrically connected to each other and thus equipotential. Such a mesh-shaped electrode can be fabricated using a typical lithography method.

In the present embodiment, the length L_(i) (one side) of the square that is the plan shape of the first conductive region 15A11 was set to 100 μm. The carrier diffusion length in the active layer 11 is estimated as 2.5 μm by calculation; the size of current spreading L_(c) is assumed to be substantially the same as the carrier diffusion length, and the interval L₂ between the mesh lines 15A2 in the second conductive region 15A12 was set to 3.0 μm, about 1.2 times as wide as the L_(c). The width L₁ of the mesh line 15A2 was set to 1.25 μm.

A result of calculating the charge density distribution formed in the charge injection region 111 of the active layer 11 in the two-dimensional photonic crystal surface emitting laser of the first embodiment is shown in FIG. 5A and FIG. 5B. In the calculation, the current spreading L_(c) was set to 2.5 μm equal to the carrier distribution length. Thus, the interval L₂ between the mesh lines 15A2 is 1.2 times as large as L_(c). FIG. 5A shows the charge density distribution at a central part 1111 (see FIG. 6) in the charge injection region 111, which corresponds to the first conductive region 15A11, and FIG. 5B shows the charge density distribution in a peripheral part 1112 the same FIG. 6) corresponding to the second conductive region 15A12. In the graphs of FIG. 5A and FIG. 5B, the position of the central part 1111 from the center is taken as the x-axis. The ordinate axis is normalized so that the value of the center (the origin of the x-axis of the graph of FIG. 5A) of the central part 1111 has one. As shown in these graphs, in the central part 1111 and the peripheral part 1112, substantially uniform charge densities are formed. Over the entire charge injection region 111, the charge density distribution where the charge density at the central part 1111 is twice as high as that of the peripheral part 1112 is formed. Formation of such a charge density distribution causes emission of light having the intensity distribution where the maximum is at the central part 1111 in the charge injection region 111. This light emission is amplified in the two-dimensional photonic crystal 123, thereby facilitating laser oscillation in the fundamental mode. This reduces useless laser oscillation in a higher mode, thereby allowing the optical output power to be increased as a whole.

To verify facilitation of laser oscillation in the fundamental mode, the threshold gain difference Δα in the first embodiment was obtained by calculation. The threshold gain difference Δα has a value obtained by subtracting the threshold gain in the next-higher mode with antinodes and nodes whose numbers are next-smaller than those in the fundamental mode from the threshold gain of the fundamental mode oscillation. The threshold gain in each oscillation mode is a value representing the intensity of laser oscillation in the oscillation mode, and means that the larger the threshold gain difference Δα, the easier the laser oscillation in the fundamental mode occurs.

First, a result of calculation of the threshold gain difference Δα in a case where the length of one side of the first electrode is L=200 μm as described above and L_(i) has different values in a range of 0 to 200 μm including 100 μm described above is shown in a graph of FIG. 7A. The abscissa axis of this graph is indicated as L_(i)/L. In the graph, the data of L_(i)/L=1 is not in the present embodiment but in a conventional two-dimensional photonic crystal laser with the first electrode made of a uniform conductive plate uniform as a whole. The data of L_(i)/L=0 shows that the first electrode is made of a mesh-shaped electrode with a uniform conductor density. According to this graph, it can be understood that in the case with the first conductive region made of the uniform conductor and the mesh-shaped second conductive region and L_(i)/L≠0 and 1, the threshold gain difference Δα is larger than the conventional case with L_(i)/L=1 and laser oscillation in the fundamental mode easily occurs. In the data shown in FIG. 7A, in a case with L_(i)/L=0.5, that is, the aforementioned case with L_(i)=100 μm and L=200 μm, the threshold gain difference Δα is the largest.

Next, a result of calculating the threshold gain difference Δα in a case with L_(i)/L of 0.5 and L of different values is shown in the graph of FIG. 7B. According to this graph, in a range larger than L of 300 μm, the larger the L is, the smaller the threshold gain difference Δα is. Meanwhile, the larger the L is, the larger the area of the entire first electrode is. Consequently, it is advantageous in view of achieving a high laser output. Based on the calculated values of Δα obtained in FIG. 7B, when laser light output power was calculated in a case of the highest L with L=600 μm (L_(i)=300 μm) among values of Δα equal to or larger than the value of Δα in the conventional example (L_(i)/L=1) in FIG. 7A, a value of 2.4 W was obtained. This calculated value of optical output power is higher than experiment values of the two-dimensional photonic crystal lasers described in Non-Patent Literatures 1 and 2 (albeit differences between the calculated value and the experiment values reside).

The case where the interval L₂ between the mesh lines 15A2 in the second conductive region 15A12 of the first electrode 15A is 3.0 μm has heretofore been described.

Results of calculating the charge density in the peripheral part 1112 in the active layer 11 in examples with different intervals L₂ are shown in FIG. 8A to FIG. 8E. The width L₁ of each mesh line 15A2 was set to 0.50 L_(c)μm. The interval L₂ was set to 0.50 L_(c)μm in FIG. 8A, 0.80 L_(c)μm in FIG. 8B, 1.20 L_(c)μm in FIG. 8C (thus, FIG. 8C is a reproduction of FIG. 5B), 1.40 L_(c)μm in FIG. 8D, and 2.00 L_(c)μm in FIG. 8E. According to these calculations, the charge density distribution in the peripheral part 1112 is almost uniform in the cases of FIG. 8A to FIG. 8C, while a slight periodical fluctuation corresponding to the period of the mesh lines 15A2 is shown but the fluctuation width is confined to be less than 5% in the case of FIG. 8D. On the contrary, in the case of FIG. 8E with the interval L₂ 1.4 times wider than the current spreading L_(c), the width of periodical fluctuation of charge density distribution is large, exceeding 10%. In the case where the widths L₁ of the mesh lines 15A2 are the same, the larger the interval L₂ is, the lower the charge density is.

FIG. 9A to FIG. 9F each show a result of calculating the charge density in the peripheral part 1112 in the active layer 11 in examples with different widths L₁ for in the case where the intervals L₂ of the mesh lines 15A2 are the same. The interval L₂ between the mesh lines 15A2 was set to 1.20 L_(c)μm. The width L₁ was set to 0.50 L_(c)μm in FIG. 9A (thus, FIG. 9A is a reproduction of FIG. 5B and FIG. 8C), 0.32 L_(c)μm in FIG. 9B, 0.20 L_(c)μm in FIG. 9C, 0.16 L_(c)μm in FIG. 9D, 0.12 L_(c)μm in FIG. 9E, and 0.08 L_(c)μm in FIG. 9F. According to these calculations, in the charge density distribution in the peripheral part 1112, the smaller the width L₁ of the mesh line 15A2 is, the lower the charge density is.

As described above, when giving the difference in the intervals L₂ and/or widths L₁ of the mesh lines 15A2 in the first electrode 15A, the lower the area ratio of the conductor (line) in the meshes is, the lower the charge density is. Therefore, by setting these interval L₂ and/or width L₁, the charge density at the corresponding position in the active layer 11 can be defined.

Instead of the unique conductor in the aforementioned example, the first conductive region 15A11 may be made of a mesh having a larger conductor area ratio than the second conductive region 15A12, for example, having the larger width L₁ of the mesh line than the second conductive region 15A12 (FIG. 10A) or having the same width L₁ but the smaller width L₂. Alternatively, three or more conductive regions (first conductive region 15A11, second conductive region 15A12, third conductive region 15A13 . . . ) may be provided so as to have the conductor area ratio gradually reducing from the center to the periphery of the first electrode 15A (FIG. 10B). As another example, a configuration may be provided in which a square-shaped first conductive region 15A11 having the highest conductor area ratio is provided at the center of a first electrode 15A, a second conductive region 15A12 having the next-highest conductor area ratio is provided in contact with the sides of the square of the first conductive region 15A11, and a third conductive region 15A13 having a lower conductor area ratio than the these two conductive regions is provided in contact with the vertexes of the square of the first conductive region 15A11 (FIG. 10C).

As shown in FIG. 11, from the center of the in-plane position of the first electrode 15A toward the outside, the area of each inter-line region 15A3 may gradually increase. In this example, the intervals L₂ between the mesh lines 15A2 have the same value irrespective of the in-plane position, and the width L₁ continuously varies so as to decrease gradually as the position is apart from the center of the in-plane position. At the center of the in-plane position, the width L₁ is the same as the interval L₂, and the inter-line regions 15A3 is not provided. According to the configuration, the conductor area ratio in the first electrode 15A substantially continuously changes so as to decrease gradually as the position is apart from the center of the in-plane position. In the charge injection region 111 of the active layer 11 into which current is injected from such a first electrode 15A, the charge density decreasing gradually from the center of the in-plane position toward the outside is formed; likewise, the intensity of light emission gradually decreases from the center of the in-plane position toward the outside. Such intensity distribution according to the position of light emission is more similar to the Gaussian distribution than the distribution in the other cases exemplified above. Consequently, laser oscillation in the fundamental mode easily occurs.

FIG. 12A and FIG. 12B each show, in a graph, a result of calculating the threshold gain difference Δα in the example of FIG. 11. It is herein assumed that the Gaussian distribution of the charge density having the width w_(p) is formed in the charge injection region 111. In FIG. 12A, L was fixed to 200 μm, and w_(p)/L was adopted as the abscissa axis of the graph. In FIG. 12B, w_(p)/L was fixed to 0.25, and L was adopted as the abscissa axis. In FIG. 12A, data with w_(p)/L=0 is not that in the present embodiment, but is that of the conventional two-dimensional photonic crystal laser where uniform charge distribution is formed in the active layer by the plate-shaped first electrode. It can be understood that the entire data in the present embodiment shown in FIG. 12A has a larger threshold gain difference Δα than the conventional case with w_(p)/L=0, and laser oscillation in the fundamental mode easily occurs. According to the graph of FIG. 12B, in a range with the value of L longer than 400 μm, the longer L is, the smaller the threshold gain difference Δα is. Meanwhile, the longer L is, the larger the area of the entire first electrode is. Thus, it is advantageous in view of increasing laser output. Based on the calculated values of Δα obtained in FIG. 12B, when laser light output power was calculated in a case of the largest L with L=2200 μm among values of Δα equal to or larger than the value of Δα in the conventional example in FIG. 12A, a value of 8 W was obtained. The calculated value of the optical output power is higher than the calculated value in a case where the charge density is adjusted only by two values using the aforementioned first conductive region 15A11 and second conductive region 15A12.

FIG. 13 shows an example of the first electrode 15A for selectively oscillating laser in a higher mode. In this example, in the square-shaped first electrode 15A, first conductive regions 15A11 were provided in the vicinity of the two vertexes on one diagonal line among the four vertexes of the square, and the remaining region was set to a second conductive region 15A12 having a lower conductor area ratio than the first conductive regions 15A11 do. This structure forms distribution where the current density is low in the vicinity of the center of the planar shape of the first electrode 15A, and the current density is high in the vicinity of the two vertexes. Injection of the current with such distribution into the active layer 11 causes light generated in the active layer 11 to become standing waves in the higher mode where a node is in the vicinity of the center and the anti-nodes are in the vicinity of the position corresponding to the two vertexes on the diagonal line in the two-dimensional photonic crystal 123. Consequently, laser in the higher mode oscillates.

Second Embodiment—Concentric-Circular Electrode

A two-dimensional photonic crystal surface emitting laser of a second embodiment uses a first electrode 15B having a configuration where ring-shaped conductors 15B1 are provided in a concentric manner, and the boundaries of the ring-shaped conductors 15B1 are insulated from each other with ring-shaped insulators 15B2 as shown in FIG. 14. A circular conductor 15B0 is provided at the center of the concentric circle. The circular conductor 15B0 and the ring-shaped conductors 15B1 are electrically connected to each other with a linear conductor 15B3. The width of the ring-shaped conductor 15B1 was set to have a ratio to the width of the ring-shaped insulator 15B2 being lower as apart from the center. Consequently, current density distribution where the density decreases as the position is apart from the center of the first electrode 15B is formed.

Third Embodiment—Divided Electrodes

In a two-dimensional photonic crystal surface emitting laser of a third embodiment, a first electrode 15C having a configuration shown in FIG. 15 was used. A first electrode 15C has a square shape in an overall view, and includes two regions that are a square-shaped first conductive region 15C11 formed in the vicinity of the center, and a second conductive region 15C12 formed around the first conductive region 15C11. An insulative region 15C21 made of an insulator linearly formed as with the four sides of a square is provided at the boundary between the first conductive region 15C11 and the second conductive region 15C12. In this structure, the first conductive region 15C11 and the second conductive region 15C12 function as the aforementioned sub-electrodes. The first electrode 15C includes a linear connective region 15C31 made of a conductor, extending from one vertex of the square of the first conductive region 15C11 on the diagonal line of the square of the second conductive region 15C12 and reaching one vertex of the second conductive region 15C12. An insulative region 15C21 is provided on both sides of the line of the connective region 15C31. The first electrode 15C can be fabricated using a typical lithography method.

The two-dimensional photonic crystal surface emitting laser of the third embodiment applies a first voltage V₁ between the first conductive region 15C11 and the second electrode 16 while applying a second voltage V₂ lower than the first voltage V₁ between the second conductive region 15C12 and the second electrode 16. During the application, the first conductive region 15C11 is connected with a first power source (not shown) via the connective region 15C31 while the second conductive region 15C12 is directly connected with a second power source (not shown) other than the first power source. The first conductive region 15C11 and the second conductive region 15C12 are electrically separated from each other by the insulative region 15C21 as described above. Thus, different voltages can be applied to the respective regions. Such voltage application forms charge density distribution with the density higher at the central part 1111 than that at the peripheral part 1112 in the charge injection region 111 of the active layer 11. Therefore emission of light having intensity distribution in which the intensity is maximum at the central part 1111 occurs from the charge injection region 111. This emitted light is amplified in the two-dimensional photonic crystal 123 to facilitate laser oscillation in a fundamental mode. Consequently, useless laser oscillation in a higher mode can be reduced, thereby allowing the optical output power to be increased as a whole.

The first electrode 15C of the third embodiment may include three or more conductive regions (the first conductive region 15C11, second conductive region 15C12, third conductive region 15C13 . . . ) from the center toward the periphery (FIG. 16). In the case of providing three or more conductive regions, all the boundaries between the conductive regions are provided with insulative regions 15C21, 15C22 . . . . In conductive regions other than the conductive region residing at the outermost, connective regions 15C31 and 15C32 . . . are provided. The conductive regions (sub-electrodes) are connected with different power sources.

REFERENCE SIGNS LIST

-   10X . . . Two-Dimensional Photonic Crystal Surface Emitting Laser -   11 . . . Active Layer -   111 . . . Charge Injection Region -   1111 . . . Central Part -   1112 . . . Peripheral Part -   12 . . . Two-Dimensional Photonic Crystal Layer -   121 . . . Base Member -   122 . . . Modified Refractive Index Regions -   123 . . . Two-Dimensional Photonic Crystal -   13 . . . Spacer Layer -   141 . . . First Cladding Layer -   142 . . . Second Cladding Layer -   15A, 15B, 15C, 15X . . . First Electrode -   15A11, 15C11 . . . First Conductive Region -   15A12, 15C12 . . . Second Conductive Region -   15A13, 15C13 . . . Third Conductive Region -   15A2 . . . Mesh Line -   15A3 . . . Inter-Line Region -   15B0 . . . Circular Conductor -   15B1 . . . Ring-Shaped Conductor -   15B2 . . . Ring-Shaped Insulator -   15B3 . . . Linear Conductor -   15C21, 15C22 . . . Insulative Region -   15C31, 15C32 . . . Connective Region -   16, 16A . . . Second Electrode -   161A . . . Window -   162A . . . Frame 

The invention claimed is:
 1. A two-dimensional photonic crystal surface emitting laser, comprising: a two-dimensional photonic crystal including a plate-shaped base member and modified refractive index regions where the modified refractive index regions have a refractive index different from that of the plate-shaped base member and are two-dimensionally and periodically arranged in the base member; an active layer provided on one side of the two-dimensional photonic crystal; and a first electrode and a second electrode provided sandwiching the two-dimensional photonic crystal and the active layer for supplying current to the active layer where the second electrode covers a range equal to or wider than the first electrode, wherein: the first electrode is a mesh of conductive material, the mesh being composed of lines of the conductive material and inter-line regions as apertures each surrounded by the lines, and the mesh is divided into a plurality of regions, each of the regions including at least one of the inter-line regions and having a mesh ratio that is different from the mesh ratio of the other regions, where each mesh ratio is defined by a ratio of an area of the lines in a unit mesh to a total area of the unit mesh, the unit mesh being composed of one of the inter-line regions and parts corresponding to a half width of the lines surrounding the one of the inter-line regions, each of the unit meshes being equally sized.
 2. The two-dimensional photonic crystal surface emitting laser according to claim 1, wherein the first electrode is formed so as to supply the current to the active layer where the distribution has a highest density at a center of an in-plane position of the first electrode.
 3. The two-dimensional photonic crystal surface emitting laser according to claim 1, wherein an interval between lines of the mesh is 1.4 times or less wider than current spreading in an in-plane direction of the active layer.
 4. The two-dimensional photonic crystal surface emitting laser according to claim 1, wherein the mesh ratio in each of the regions decreases as a position is apart from a center of an in-plane position of the first electrode.
 5. The two-dimensional photonic crystal surface emitting laser according to claim 4, wherein a region including the center in the first electrode consists only of the conductive material.
 6. The two-dimensional photonic crystal surface emitting laser according to claim 1, wherein: the first electrode is formed so as to supply the current to the active layer with a different density depending on an in-plane position on the first electrode to cause emission of light, the light is amplified in the two-dimensional photonic crystal and causes laser oscillation, and the two-dimensional photonic crystal surface emitting laser is configured to emit the oscillating laser light from the second electrode side.
 7. A two-dimensional photonic crystal surface emitting laser, comprising: a two-dimensional photonic crystal including a plate-shaped base member and modified refractive index regions where the modified refractive index regions have a refractive index different from that of the plate-shaped base member and are two-dimensionally and periodically arranged in the base member; an active layer provided on one side of the two-dimensional photonic crystal; and a first electrode and a second electrode provided sandwiching the two-dimensional photonic crystal and the active layer for supplying current to the active layer where the second electrode covers a range equal to or wider than the first electrode, wherein: the first electrode is a mesh of conductive material, the mesh being composed of lines of the conductive material and inter-line regions as apertures each surrounded by the lines, and a mesh ratio of the mesh changes continuously depending on a position, where the mesh ratio is defined by a ratio of an area of the lines in a unit mesh to a total area of the unit mesh, the unit mesh being composed of one of the inter-line regions and parts corresponding to a half width of the lines surrounding the one of the inter-line regions, each of the unit meshes being equally sized.
 8. The two-dimensional photonic crystal surface emitting laser according to claim 7, wherein the mesh ratio decreases as a position is apart from a center of an in-plane position of the first electrode.
 9. The two-dimensional photonic crystal surface emitting laser according to claim 7, wherein: the first electrode is formed so as to supply the current to the active layer with a different density depending on an in-plane position on the first electrode to cause emission of light, the light is amplified in the two-dimensional photonic crystal and causes laser oscillation, and the two-dimensional photonic crystal surface emitting laser is configured to emit the oscillating laser light from the second electrode side.
 10. A two-dimensional photonic crystal surface emitting laser, comprising: a two-dimensional photonic crystal including a plate-shaped base member and modified refractive index regions where the modified refractive index regions have a refractive index different from that of the plate-shaped base member and are two-dimensionally and periodically arranged in the base member; an active layer provided on one side of the two-dimensional photonic crystal; and a first electrode and a second electrode provided sandwiching the two-dimensional photonic crystal and the active layer for supplying current to the active layer where the second electrode covers a range equal to or wider than the first electrode, wherein the first electrode includes a circular conductor having a conductive material at a center of the circular conductor, ring-shaped conductors arranged concentric-circularly around the circular conductor, ring shaped insulators insulating the conductors, a ratio of a width of the ring shaped conductors to a width of the ring shaped insulators is higher in a center portion of the ring shaped conductors than in an outside portion of the ring shaped conductors, and a single linear conductor for electrically connecting the circular conductor and ring-shaped conductors.
 11. The two-dimensional photonic crystal surface emitting laser according to claim 10, wherein: the first electrode is formed so as to supply the current to the active layer with a different density depending on the in-plane position on the first electrode to cause emission of light, the light is amplified in the two-dimensional photonic crystal and causes laser oscillation, and the two-dimensional photonic crystal surface emitting laser is configured to emit the oscillating laser light from the second electrode side. 